Conventionally, in an image forming device, paper sheets that have been printed are stacked on a paper discharge tray. The user who initiated the printing usually takes the paper sheets that are stacked on the paper discharge tray soon after the paper sheets are discharged from the image forming device. However, if the user procrastinates taking the paper sheets from the paper discharge tray, the printed paper sheets will be left unattended on the paper discharge tray for a long period of time. In situations where the image forming device are shared by a plurality of users, such as offices, schools, shops, and so on, information printed on the unattended paper sheet may be disclosed to users who did not initiate the printing. The confidentiality of the information printed on the unattended paper sheet would be compromised in such situations.